RocketMotorTwo is a hybrid rocket engine being developed for the Scaled Composites SpaceShipTwo suborbital spaceplane. The engine is being designed and built by Sierra Nevada Corporation (SNC), and was first ignited in flight during a SpaceShipTwo test launch in April 2013.
RocketMotorTwo is a throttleable hybrid rocket engine fueled with hydroxyl-terminated polybutadiene (HTPB) and nitrous oxide – sometimes referred to as an N2O/HTPB engine – providing 60,000 pounds-force (270 kN) of thrust.
As of March 2013[update], Sierra Nevada has performed over 300 hybrid rocket test firings. The company also developed the rocket engine for the first private spacecraft to reach space, SpaceShipOne, which won the Ansari X-Prize in 2004. Sierra Nevada is furthermore developing a similar hybrid engine, the Dream Chaser rocket engine.
|This section requires expansion. (April 2013)|
Hot-fire ground tests
Scaled Composites performed a series of subscale rocket hot-firings between June 2005 and April 2009, before choosing a full-scale rocket motor design. By December 2012, 21 full-scale hot-fire ground tests had been carried out on RocketMotorTwo. On 20 June 2012, the first hot-fire test under the control of SpaceShipTwo's proprietary Rocket Motor Controller (RMC) was successfully conducted.
Test program accident
On 26 July 2007, during the early rocket testing phase, an explosion occurred during a propellant flow test at the Mojave Air and Space Port. The test included filling the oxidizer tank with 4,500 kg (10,000 pounds) of nitrous oxide, followed by a 15-second cold flow injector test. Although the tests did not ignite the gas, three employees were killed and three injured, two critically and one seriously, by flying shrapnel.
Commencement of flight testing
In July 2012, Virgin Galactic, the owner of VSS Enterprise – the first SpaceShipTwo spaceplane built – announced that RocketMotorTwo was fully qualified for powered flight, and that the ground test program was substantially complete. SpaceShipTwo conducted its first gliding test flight with the engine fully installed in December 2012. Additional ground rocket tests continued into March 2013 as the company prepared for powered test flights.
VSS Enterprise conducted its first powered flight on 29 April 2013, marking the first flight test of RocketMotorTwo. The test was a 16-second burn as planned, and was ignited at an altitude of 47,000 feet (14,000 m), shortly after Enterprise was released from its WhiteKnightTwo carrier aircraft and the pilots cross-checked data and verified stable control. The RM2 control system opened the main oxidizer valve and fired the igniters inside the fuel case. The burn was completed at an altitude of 55,000 feet (17,000 m), by which point Enterprise was supersonic, achieving Mach 1.2 (790 mph; 1,300 km/h). SNC stated after the test that "The rocket motor ignition went as planned, with the expected burn duration, [and] good engine performance."
2014 Change of fuel
In May 2014, Virgin Galactic announced a change to the fuel to be used in the SpaceShipTwo rocket engine. Rather than the rubber-based HTPB—HTPB engines had experienced serious engine stability issues on firings longer than approximately 20 seconds—the engine will now use a type of plastic called thermoplastic polyamide as the solid fuel. The plastic fuel is projected to have better performance (by several unspecified measures) and will allow SpaceShipTwo to make flights to a higher altitude.
As of May 2014[update], the new engine formulation has already completed full-duration burns of over 60 seconds in ground tests on an engine test stand. However, four additional ground tests of the polyamide-fueled engine are anticipated before the SpaceShipTwo flight test could resume with the new-fuel rocket motor.
The hybrid rocket motor and its oxidizer valve system are produced in SNC’s manufacturing facility in Poway, California in conjunction with Scaled Composites. In 2013, the Poway facility was reported to be "currently producing motors for both SpaceShipTwo and SNC’s own Dream Chaser orbital crew vehicle".
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